Although scoring systems primarily rely on stone characteristics such as size, number, and location in the collecting system, they have different predictive values due to the variety of other parameters they include. In the RUSS and R.I.R.S. scoring systems, anatomical features such as infundibulopelvic angle (IPA) and infundibular length were also evaluated using urography imaging [6, 10]. In Polat, R.I.R.S., and Ito’s scoring systems, not only the diameter but also the area and volume were taken into account for the size of the stone, aiming to measure the stone burden more precisely [12]. Stone density significantly affects stone fragmentation, and this parameter is included in the R.I.R.S., T.O.HO., S.T.O.N.E., and Polat scoring systems [7, 10,11,12] The S-RESC scoring system, initially described for predicting SFR after percutaneous nephrolithotomy (PNL), was later adapted for fURS [8]. It is based only on the location of the stone in the collecting system, without considering parameters like stone size, number, density, and presence of hydronephrosis. As a result, it has been criticized for its limited predictive value.

The parameters in the study by Resorlu et al. were reported to be independent predictors for SFS, but the AUC for the RUSS score was not mentioned [6]. In a subsequent external validation study, it was reported to have an AUC value of 0.735 [13]. The RUSS scoring system has many limitations. Urography is required for IPA measurement, and the measurements require experience, leading to an obstacle in achieving a standard measurement. In addition, although Resorlu et al. stated that they also evaluated patients with renal anomalies, only 2 patients in their study group had renal malformations. Therefore, this study does not present sufficient data to make judgements about patients with renal anomalies. Nevertheless, it was found to be an independent predictor of SFS in validation studies.

The S.T.O.N.E. scoring system, which can be applied relatively easily, evaluates stone characteristics and the degree of hydronephrosis, and has limited reliability with an AUC value of 0.63. The validity of this scoring system is limited because of the retrospective design of the study, and the small proportion of patients with high scores in the study group [7]. However, the external validation study conducted by Selmi et al. described high predictive values for the S.T.O.N.E. scoring system in terms of SFS (AUC: 0.725) [14].

The S-RESC scoring system does not include the characteristics of the stone, except its localization in the collecting system. When defined by Jung et al., it was reported to have a high predictive value for SFS (AUC: 0.806) [8]. However, it does not evaluate any parameters such as stone size, number, density, and presence of hydronephrosis. If stones of different sizes and densities are localized in the same region of the kidney, the S-RESC system would still propose the same results, which is why it has been criticized in many studies. In external validation studies; while Polat et al. reported that it had no predictive value (AUC: 0.582) [12]. Selmi et al. stated that it had a high predictive value for SFS (AUC: 0.755). However, in the regression analysis of the same study, it was seen that the S-RESC scoring system was not an independent predictor of SFS [14].

The R.I.R.S. scoring system, which includes all features of the stone and the parameters related to the anatomical structure of the kidney, was defined by Xiao et al. in 2017. It provided high predictive values for SFS in the internal validation (AUC 0.828 for the 1st postoperative day, 0.904 for the 1st postoperative month) [10]. However, external validation studies could not provide similar results [15]. This may probably be related to a lack of experience or lack of standardized measurements of IPA, infundibular length, or stone density.

Recently, Polat et al. reported a new nomogram in their study, in which the surface area of the stone, the number of stones, the density of the stone, and the localization of the stone in the collecting system are identified as independent predictors of SFS. It differs from other scoring systems in only evaluating large stones between 2 and 4 cm. A high predictive value was reported for SFS (AUC: 0.802) [12]. However, in the same study, it was not determined by regression analysis whether the nomogram was an independent predictor of SFS. This study had some limitations, such as being a retrospective study conducted with a small number of patients, and the exclusion of patients with musculoskeletal abnormalities and complex stones. Most importantly, it is frustrating why fURS was performed on patients who would be candidates for PNL as first-line treatment according to the guidelines. Treatment of large kidney stones with fURS may result in a decrease in SFR and reoperation may be required. Its reliability in < 2 cm stones is unclear. Larger external validation studies may shed light on these questions.

In 2015, Ito et al. evaluated the data of 310 patients who underwent fURS and developed a nomogram utilizing parameters such as stone volume, presence of lower pole stone, surgeon’s experience, number of stones, and presence of hydronephrosis, which they found to be independent predictors for SFS. This scoring system, called Ito’s nomogram, was proposed to have a high predictive value, with an AUC value of 0.87. Higher scores result in an increased possibility of achieving SFS [9]. This nomogram is the only scoring system that takes surgeon’s experience into consideration as an independent predictor of SFS. Stone density was also examined in the study, but it was not included in the nomogram as it was not detected as an independent predictor. The limitation of this nomogram is that it does not evaluate renal malformations. It showed a low predictive value for SFS in the external validation study (AUC: 0.658), and the regression analysis showed that Ito’s nomogram was not an independent predictor of SFS [12]. The result of this external validation study could be confusing, as the study group consisted of patients with large stones (2–4 cm), who were less likely to achieve SFS.

T.O.HO. is a practical scoring system that consists of 3 parameters. Patients were evaluated based on stone length, location, and density. Although fewer parameters are evaluated, they provide high predictive values, with an AUC of 0.83 [11]. The T.O.HO. scoring system also showed a high predictive value for SFS in its external validation study (AUC: 0.758). In the same study, it was observed that a higher predictive value could be achieved with the modified T.O.HO. scoring system, obtained by adding stone volume to the parameters (AUC: 0.821) [16]. If similar success rates are determined with external validation studies, it seems promising in the future as a simple and practical scoring system.

This study aimed to externally validate Ito’s nomogram and T.O.HO. scoring systems, which are highly reliable and can be easily applied using preoperative CT. We demonstrated that both scores are good predictors of SFR after fURS with high accuracy rates (AUC: 0.792, AUC: 0.744, respectively). Additionally, in our study, by regression analysis, stone size, number of stones, lower pole localization, as well as Ito’s nomogram and the T.O.HO. scoring system, were found to be independent predictors of SFS.

As well as achieving SFS, success of the surgery also requires completion without complications or with an acceptable level of complications. The biggest shortcoming of these scores is that none of the scoring systems described in this article have examined their predictive strength for surgical complications. In R.I.R.S., the authors stated that the score correlated with the duration of surgery and could therefore be used to predict postponement of surgery to a second session and prevent complications [10]. In an external validation study by Bozkurt et al., RUSS, R.I.R.S., and Ito’s nomogram were able to accurately predict complications [17].

Almost all scoring systems have been reported to have a high predictive value in the studies in which they were defined. However, external validation studies show that this is not the case, and even meta-analyses show that there is a serious heterogeneity in the results. It is emphasized that this heterogeneity is due to the lack of standardized measurements or inconsistencies in the definition of surgical success. When the studies thought to cause heterogeneity were excluded, the S.T.O.N.E. scoring system was found to have the highest AUC value with 0.771 compared to S-RESC, R.I.R.S., and RUSS systems (0.709, 0.704, and 0.669, respectively). In the same meta-analysis, homogeneity regarding Ito’s nomogram could not be achieved by the exclusion of studies. In this meta-analysis, it was reported in the conclusion section that no scoring system was superior to other scoring systems in pairwise analysis [15].

An ideal scoring system is expected to cover all factors affecting success and complications, while at the same time being easily applicable. In our study, we demonstrated that Ito’s nomogram and T.O.HO. scoring system provides high accuracy rates with a reasonable number of parameters that can easily be evaluated. Although these scores provide a prediction for surgical success, the final surgical decision should be based on the patient’s condition and expectations.

Due to limited data in our study, the relationship between complications could not be evaluated. The limitations of the study include the retrospective collection of data and the use of X-ray for residual stone control. In other centers, the use of computed tomography for residual stone control is essential for validation of the study.